This invention relates in general to transducers and in particular to optical sensors for detecting and measuring physical parameters by converting such parameters into modulated light signals.
Optical sensors have been developed to replace traditional electrical sensors to measure physical variables not possible with electrical sensors and to provide better performance. Other reasons for preferring optical over electrical signal sensing and transmission is the elimination of electromagnetic interference and inherent electrical isolation.
One type of conventional optical sensors employs fiberoptic techniques. In U.S. Pat. No. 4,071,753 to Fulenwider et al., the ends of an input and an output optical fiber are aligned and light is transmitted from one fiber to the other. Various means (apparently mechanical means) couple the end of the output optical fiber to a mechanical or acoustic source. The mechanical or acoustic signal from the source varies the optical coupling coefficient between the two fibers so that by measuring such coefficient, the mechanical or acoustic information can be measured. Fuller in U.S. Pat. No. 4,419,895 discloses an angular accelerometer comprising a pair of cantilevered optical fibers which are parallel but somewhat misaligned. Angular movement modulates the optical signals coupled between the two optical fibers. By measuring such modulations, the angular accelerations are detected. In the above-described type of transducers, the optical sensor comprises two optical fibers optically coupled. The optical coupling coefficient between the two fibers varies with the physical parameter to be measured, so that by measuring such coefficient, the parameter can be detected and measured.
In another type of optical sensors the physical parameter to be measured modulates the vibratory motion of a transducer element. Such modulation changes the intensity of light coupled between the ends of two optical fibers so that by measuring such changes the physical parameter can be detected and measured. Such type of transducers is disclosed in U.S. Pat. No. 4,345,482 to Adolfsson et al. The transducer element may be a vibrating spring, a ferroelectric or a piezoelectric element. In U.S. Pat. No. 4,379,226, Sichling et al. disclose a somewhat analogous optical sensor. The transducer element used in Sichling et al. is a leaf spring torsional rotational device, a ferromagnetic element attached to a filament or a piezoelectric element such as a quartz crystal. Sichling et al. also disclose an arrangement in which a mirror is attached to a transducer element which reflects light received from the input optical fiber towards the output optical fiber to accomplish the light coupling between the two fibers.
Piezoelectric quartz resonators have been disclosed in more detail in other patents such as U.S. Pat. No. 3,561,832 to Karrer et al. Such resonators exhibit changes in frequency when subjected to stress and may be used to measure stress. A ferromagnetic tuning fork is used for measuring gas pressure as disclosed in U.S. Pat. No. 3,902,355 to Weisser. The resonance frequency of the tuning fork is altered by changes in the gas pressure so that by mesuring the modulations in resonance frequency of the tuning fork the gas pressure can be measured. In U.S. Pat. No. 4,279,028, Lowdenslager et al. disclose the use of an unsealed tuning fork quartz crystal for measuring atmospheric pressure change using the same principle. In an article entitled "Using the X-Y Flexure Watch Crystal As A Pressure-Force Transducer," Proceedings of 31st Annual Frequency Control Symposium 1977, U.S. Army Elect Command, A. Genis et al. disclose the use of an X-Y flexure watch crystal for measuring pressure by the same principle. None of the above disclosures on the quartz crystal, however, appear to teach the use of the optical properties of the quartz crystal simultaneously with its piezoelectric properties.
An optical sensor utilizing a grating structure connected to a diaphragm in a hydrophone is disclosed by W. B. Spillman in the article entitled "Multimode Fiber-Optic Hydrophone Based On A Schlieren Technique", Vol. 20 No. 3 of Applied Optics, February 1981. The grating structure is placed between the ends of two optical fibers to vary the light coupled between the two fibers. Thus an acoustic signal which causes the diaphragm to vibrate will cause a corresponding variation in the light coupled between the two fibers. The acoustic signal is then measured by the modulation in the light coupling between the fibers. Various hydrophones for measuring acoustic waves are disclosed in the above-referenced article by W. B. Spillman.
Piezoelectric sensors have been used to detect machinery vibrations such as engine knock in an internal combustion engine. Such an application is described in U.S. Pat. No. 4,349,404 to Hamisch et al. The high impedance associated with this type of piezoelectric acceleration sensors are prone to electrical noise such as that generated by engine ignition systems. An accelerometer employing a brass cantilevered beam is described by Sorf et al in the paper "Tilting-mirror Fiber-optic Accelerometer," Applied Optics, Vol. 23, No. 3, Feb. 1, 1984.
In addition to the detection and measurement of force, pressure, stress and acoustic waves, quartz crystal vibrators have been used to measure temperature. One such instrument, Model HP 2801 manufactured by Hewlett-Packard, Palo Alto, Calif., is based upon a crystal thermometer described by Hammond in U.S. Pat. No. 3,423,609. Statek of Orange, Calif., manufactures a quartz thermometer tuning fork (Model TS-2). In U.S. Pat. No. 4,398,115 Gagnepain et al. also describes the temperature indicating quartz crystal plate.
Quartz crystal detectors have also been used as highly sensitive micro-balance detectors. Quartz crystals coated with a tacky substance or a substance which selectively absorbs and/or adsorbs a particular chemical or molecule increases the mass of the oscillator causing a decrease in its resonance frequency. Dorman in U.S. Pat. No. 3,561,253 describes a particle detector system using an oscillating quartz crystal plate. Chuan in U.S. Pat. No. 3,715,911 measures the mass of atmospheric particulate matter with a vibrating quartz crystal plate micro balance. Crystals coated with a selectively absorbent coating is described in U.S. Pat. Nos. 4,111,036 to Frechette et al. (for detecting sulfur dioxide), 4,193,010 to Kompanek and in an article entitled "Analysis of Environmental Pollutants Using a Piezoelectric Crystal Detector," in Intern. J. of Environ. Anal Chem. 1981 Vol. 10, pp. 89-98 by Guilbault.
In all the above-described applications for measuring a physical parameter using the quartz crystal, the crystal is used as a non-optical vibrating detector which provides electrical signals for the detection and measurement of the physical parameters. These electrical signals are then processed by electronic circuitry. Such quartz crystal detectors are subject to electromagnetic interference which is undesirable.
Techniques for fabricating quartz crystal resonating structures such as microlithographical, chemical and other manufacturing processes are well known to those skilled in the art. Such processes are disclosed for example in U.S. Pat. Nos. 3,683,213 and 3,969,640 both issued to Staudte. Also well known in the art are the techniques for defining the cut of quartz crystal resonators to provide certain temperature-frequency characteristics, to tune their resonance frequencies, and to define the location and configuration and excitation electrodes to cause movements such as the flexure and twisting (torsional) modes of tuning forks. These movements can be at the fundamental and/or overtone frequencies of the crystal. Examples of these techniques are disclosed in U.S. Pat. Nos. 3,969,641 by Oguchi et al.; 4,377,765 by Koqure et al; 4,382,204 by Yoda and 4,384,232 by Debely. In addition, quartz crystal tuning forks may be vibrated in directions normal to the plane containing both tines of the tuning fork. Such techniques are also well known.